Semiconductor devices are commonly found in modern electronic products. Semiconductor devices vary in the number and density of electrical components. Discrete semiconductor devices generally contain one type of electrical component, e.g., a light emitting diode (LED), photodiode, small signal transistor, resistor, capacitor, inductor, or power metal-oxide-semiconductor field-effect transistor (MOSFET). Integrated semiconductor devices typically contain hundreds to millions of electrical components. Examples of integrated semiconductor devices include microcontrollers, microprocessors, charged-coupled devices (CCDs), solar cells, and digital micro-mirror devices (DMDs).
Semiconductor devices perform a wide range of functions such as signal processing, high-speed calculations, transmitting and receiving electromagnetic signals, controlling electronic devices, transforming sunlight to electricity, and creating visual projections for television displays.
Semiconductor devices are found in the fields of entertainment, communications, power conversion, networks, computers, and consumer products. Semiconductor devices are also found in military applications, aviation, automotive, industrial controllers, and office equipment.
Optical fibers are commonly used to transmit signals between semiconductor devices that are remote from each other. A light emitting diode (LED), laser diode, or another electronically controllable light source is used to generate a light wave into a fiber. The fiber guides the light wave from the source device to a destination device. The destination device includes a photodiode that converts the optical signal into an electrical signal for processing. Commonly an avalanche photodiode (APD) is used, and a transimpedance amplifier (TIA) amplifies the electrical signal for use by semiconductor devices.
FIG. 1a illustrates a simplified optical receiver circuit. A light signal is received from an optical fiber and directed at photodiode 20. Photodiode 20 is an APD in some embodiments. Photodiode 20 modulates current source 24 drawing current from voltage source 26 to ground node 28, and thus controls a input signal to trans-impedance amplifier (TIA) 30. TIA 30 outputs a differential pair signal as TIA output 40 that is coupled to a transceiver for further processing.
FIG. 1b illustrates a portion of a transceiver circuit for an optical system. TIA output 40 is coupled to an input of receiver limiting amplifier (RLA) 50. RLA 50 outputs a constant power output signal 60 to other transceiver logic that converts the signal to digital data. In order to determine when the input signal has been lost, the transceiver includes a loss of signal (LOS) circuit.
In the past, LOS circuits have been made up of a reference signal generator 70 that outputs a reference signal 72 to a LOS amplifier 74. Reference signal 72 is a signal similar to TIA output 40 at approximately the minimum magnitude level suitable for input to RLA 50. Reference signal 72 operates as a threshold, typically 5 millivolts (mV) peak-to-peak. LOS amplifier 74 is made as nearly identical to RLA 50 as reasonably possible. A comparator 78 is used to compare the output of RLA 50 and LOS amplifier 74. If the magnitude of data output signal 60 falls below the magnitude of the output from LOS amplifier 74, comparator 78 asserts LOS signal 80 to notify the system of a loss of signal.
RLA 50 in the main data path of optical receivers usually includes large amplifier stages and draws large electrical currents. Because RLA 50 is in the main data path, the data stream cannot be interrupted to check for loss of signal. Therefore, a second identical amplifier 74 is provided to do the LOS comparison. Having two identical amplifiers allows the reference to be compared against the received data without interrupting the main data path, but requires significant space on an optical receiver chip and increases overall current draw dramatically. Moreover, comparator 78 represents a significant load on RLA 50, thus reducing the performance of the RLA. Therefore, a need exists for a LOS circuit with a reduced circuit footprint and electrical current requirement.